U.S. forces struck Iranian rocket launchers on the Strait of Hormuz in their first military action in a month on Sunday, breaking a lull in fighting during an intermittent war that has lasted more than six months.

Iran swiftly vowed to retaliate for what it called a deadly attack.

Forces with the Revolutionary Guard Corps were observed preparing to launch rockets with sea mines into the strait, according to Tim Hawkins, a spokesperson for the U.S. Central Command.